1. Field of the Invention
The invention relates generally to optical coherence tomography, specifically to the use of a cross-dispersed, echelle configuration, spectrometer in a spectral domain optical coherence tomography system.
2. Description of Related Art
Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. In recent years, it has been demonstrated that spectral domain OCT has significant advantages in speed as compared to time domain OCT. In spectral domain OCT (SD-OCT) the optical path length difference between the sample and reference arm is not mechanically scanned but rather the interferometrically combined beam is sent to a spectrometer in which different wavelength components are dispersed onto different photodetectors to form a spatially oscillating interference fringe (Smith, L. M. and C. C. Dobson (1989). “Absolute displacement measurements using modulation of the spectrum of white light in a Michelson interferometer.” Applied Optics 28(15): 3339-3342). A Fourier transform of the spatially oscillating intensity distribution can provide the information of the reflectance distribution along the depth within the sample. As there is no mechanical depth scanning, acquisition of light reflection along a full depth range within the sample can be achieved simultaneously, and consequently, the speed of obtaining a full depth reflection image is substantially increased as compared to time domain OCT (Wojtkowski, M., et al. (2003). “Real-time in vivo imaging by high-speed spectral optical coherence tomography.” Optics Letters 28(19): 1745-1747; Leitgeb, R. A., et al. (2003). “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography.” Optics Letters 28(22): 2201-2203). In addition, as the light reflected from the full depth range within the sample is fully dispersed over many photodetectors, the shot noise for each photodetector is substantially reduced as compared to the time domain OCT case, and hence the signal to noise ratio can also be substantially increased (Leitgeb, R. A., et al. (2003). “Performance of Fourier domain vs. time domain optical coherence tomography.” Optics Express 11(8): 889-894; De-Boer, J. F., et al. (2003). “Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography.” Optics Letters 28(21): 2067-2069; Choma, M. A., M. V. Sarunic, et al. (2003). “Sensitivity advantage of swept source and Fourier domain optical coherence tomography.” Optics Express 11(18): 2183-2189).
SD-OCT systems seek to achieve high axial resolution at moderate scan depths or moderate axial resolution at large scan depth. For example, in current applications to retinal diagnosis, retinal images with approximately 2 microns resolution over a depth of approximately 2 mm are desired (Ko, T. H., J. G. Fujimoto, et al. (2005). “Comparison of Ultrahigh- and Standard-Resolution Optical Coherence Tomography for Imaging Macular Pathology.” Ophthalmology 112(11): 1922-1935). In current applications to prescription of intraocular lenses (Hitzenberger, C. (1991). “Optical measurement of the axial eye length by laser Doppler interferometry.” Invest. Ophthalmol. Vis. Sci. 32(3): 616-624), all optical interfaces within the eye (up to 30 mm long) need to be located with a resolution of around 30 microns. For these two applications the desired ratio of scan depth in tissue to axial resolution is about the same (30 mm/30 microns vs. 2 mm/2 microns) and higher values of the ratio give further improvements to the images. In straightforward SD-OCT techniques the information in one axial scan is encoded in the spectrum (Leitgeb, R. A., et al. Optics Express 11(8): 889-894). A large ratio of scan depth to axial resolution implies large information content within one axial scan, which in turn requires a large number of pixels along a linear detector array; the examples above require at least 4000 pixels. Given the typical 10-micon spatial optical resolution of a spectrometer operating at wavelengths near 1 micron (typical for biological applications) resolving the information content in the spectrum requires a spectrum length of approximately 40 mm. Such arrays are not commonly available and even if such an array would be made available, the optics layout would necessarily be very large even in a Littrow arrangement, causing thermal problems for example.
A solution to this problem would be the use of a cross-dispersed echelle spectrometer. In such a configuration, the light emerging the fiber or pinhole is collimated before it hits the echelle grating. An echelle is a coarsely ruled grating (for example of the order of 50 grooves per millimeter) that is designed to be used in high orders of diffraction, denoted by m, typically m=30 or higher in order to achieve the desired spectral resolution. In such a configuration, the spectral width of the source (Δλ>130 nm for high resolution OCT) is diffracted into several orders, and these orders will overlap. That is, at a given angle of diffraction, several discrete wavelengths within the bandwidth of the source will be diffracted at that angle, each wavelength being diffracted in a different order of diffraction. Thus if the dispersed spectra would fall onto a linear detector array, each detector in the array would receive several wavelengths, each wavelength from a different order of diffraction. To separate the light from these different orders of diffraction, an optical element providing relatively low dispersion can be placed after the echelle with its dispersion direction perpendicular to the echelle grating's dispersion direction. A low-dispersive element used in this way is called cross-disperser. The different diffraction orders are now spatially separated, and can now be imaged with a 2-dimensional area detector array or with a stack of linear detector arrays. Such spectrometers can be built up in the classical way; that is, collimating the light emerging the fiber before it falls onto the echelle grating, re-imaging the dispersed light with a lens system onto a two-dimensional detector array after it has been cross-dispersed.
Cross-dispersed echelle spectrographs have been used mostly in the field of astronomy, where extreme wavelength resolutions are required, for example R>100,000 and where the recent availability of 2D infrared detector arrays allows the imaging of multiple, cross-dispersed echelle orders (see e.g. McLean et al., 1998, SPIE Proceedings Vol. 3354, pp 566). There, the echelle grating is also used in a Littrow arrangement in order to save space, because it needs to be cryogenically cooled. However, these prior art echelle spectrographs are typically geared towards the highest possible spectral resolutions, whereas spectral-domain OCT typically requires resolutions in the range of 2,000<R<10,000.
This cross-dispersed spectrometer can achieve the benefits of compactness and stability of alignment described in co-pending application Ser. No. 11/196,043, filed on Aug. 3, 2005 (incorporated herein by reference) by using the echelle grating in the Littrow configuration. In order to separate the diffracted beams from the input beam, the echelle grating can be tipped to produce conical diffraction. The conical diffraction creates certain distortions and non-linearities in the focused beam. These problems are described in greater detail below with respect to FIG. 3. One aspect of the subject invention is to provide optical correction for such distortions.